If you got your hands on a fish eye lens, youll be stunned and how the optics in the lens react.. This is not a "fish eye" lens, its not even a lens, more like a fitler that enhances the lens into a fish eye effect. yes effect and not a real fish eye..
I'd say give it a go! I got a set of filters that act as mini macro lens for $10 and they are actually pretty incredible. You'll never know if you don't try. Read all the reviews on it you can, and see if there's any customer images they've shot with that lens. Best of luck to you!
It's $39 - it could be too good to be true and it could be a decent lens to play around with. Regardless it's $40 - buy it and if you want a better fisheye in the long run then you can save up for it. You might get the lens and realize you hate shooting fisheye - better to be out $40 (which you could probably sell later) than $200+.
That being said it's also rated 4/5 stars on Amazon so enough people have used it and have been happy with it that it can't be totally awful.